With appropriate electrical loading, photovoltaic solid state semiconductor devices, commonly referred to as solar cells, convert sunlight into electrical power by generating both current and voltage upon illumination. The current sources in a solar cell are charge carriers that are created by the absorption of photons by the solid state semiconductor device. These photogenerated carriers are typically separated and collected by the use of PN or PIN junctions in the solid state semiconductor devices. The operational voltage of photovoltaic devices is limited by the dark diode current characteristics of the underlying PN or PIN junction(s). Thus improving the power output performance of any solid state solar cell entails simultaneously maximizing absorption and carrier collection while minimizing dark diode current.
Photovoltaic (PV) technologies that convert sunlight directly into electricity hold great promise as a sustainable, environmentally friendly energy source for the 21st century. However, power generation with conventional, crystalline-based silicon technologies is limited both in terms of performance and manufacturing costs. Various organic and inorganic thin film solar cell technologies have been developed that promise to lower the costs of photovoltaic power, but are much lower in solar electric power conversion efficiency. Current thin film technologies are thus a poor use of land resources, and are not suitable for applications constrained in area.
Compared with crystalline wafer cells, thin film solar cells require only a fraction of the semiconductor material and can be deposited on lower cost glass and flexible substrates, thus offering significantly lower manufacturing costs. While a number of different materials can be used to construct thin film cells, silicon-based materials are particularly attractive due to their abundance and lack of toxicity. However, the conversion efficiency of the best silicon thin film solar cells is in the range of 8-12%, compared to almost 25% for state-of-the-art wafer silicon devices. Because current thin-film cells have a significantly lower efficiency than conventional silicon modules, they are limited to installation sites that are not constrained by available real estate area and may not produce enough electrical power per unit area to be economically viable in many locations. Therefore, it is desirable to provide design strategies and processes that can maximize both the photocurrent generating capability of silicon and other thin film solar cell devices and their voltage output.
The operating voltage of a semiconductor solar cell is generally dictated by the underlying dark diode current of the device. The dark diode current of semiconductor devices is composed of several different components, all of which are dependent upon the energy gap of the material used in the active junction of the device. Typically, each cell in a solar cell consists of one type of material, and the energy gap of that material influences both the current and voltage output of the device. Lower energy gap material can enhance the current generating capability, but typically results in a lower operating voltage. Therefore, it is desirable to provide a device and a method that can harness the current generating capabilities of narrow energy gap material while also maintaining a high operating voltage.